1. Field of the Invention
The present invention relates to a method and to a device for emitting radial seismic waves in a material medium such as the subsoil, by electromagnetic induction, used notably for generating seismic waves in cased wells or in a water body.
2. Description of the Prior Art
There are different types of seismic sources suited to emit in wells. They involve:
explosions: a point or an elongated charge or detonating cord helically wound on a rigid spindle,
electric sparks (a breakdown between electrodes in water or wire explosion under the effect of an electric discharge, etc.),
vertical mechanical shock of a mass falling or thrown onto an anvil secured to a packer, which produces a vertical shear on the well wall mainly generating S waves,
or a horizontal shock, in a radial direction, of a mass radially driven by hydraulic or electromagnetic device and that strikes the wall of the well at one point.
Controlled vibrational sources of piezoelectric or magnetostrictive type, coupled (or not) with the well wall, which emit a single frequency or signals that are coded or frequency-modulated by a ramp, are also used to create seismic waves in wells.
These sources can be used in a cased or an uncased well. In the case of cased wells, their efficiency is affected by the stiffness of the casing which limits the stress applied to the surrounding medium.
The device according to the invention uses a well-known physical principle, i.e. the motive action provided by an impulsive magnetic field already used in other applications, for example:
the making of marine acoustic sources where the repulsion of two metal disks surrounding a flat coil fed by an electric shock generator (plane structure) is used,
the making of electromagnetic shutters for electromagnetic radiations (optical or X-ray spectrum) consisting of a thin metal tube placed in line with and inside a coil whose impulsive magnetic field causes collapse,
plasma acceleration, etc.
The device according to the invention finds applications notably for operations of seismic prospecting or monitoring of the subsoil wherein seismic wave emission is conventionally triggered and the waves reflected by the formation discontinuities are recorded by means of seismic receivers (geophones, hydrophones) in order to form seismograms. The device can be used for example to generate seismic waves in cased or uncased wells or in a water body.
The method according to the invention allows generation of radial elastic waves in a material medium. The method radially expands at least part of the wall of a metal tube in contact with the medium under the effect of a magnetic pressure generated by electromagnetic induction, with emission in the medium of the elastic waves created in the medium under the effect of this expansion.
The device according to the invention allows generation of radial elastic waves in a material medium. It essentially comprises a metal tube in contact with the medium and a motive device arranged inside the tube to exert either an isotropic magnetic pressure on all of the wall of the tube, thus causing a radial expansion of the wall of the metal tube, or an anisotropic magnetic pressure on the wall of the tube, causing an (anisotropic) radial expansion of only part of the wall of the metal tube.
According to a first embodiment, the motive device comprises a coil formed for example on an insulating spindle with a constant or a variable winding pitch that can increase from the central part to the ends of the coil so as to increase the dipolar radiation along the axis of the coil, or decrease from the central part to the ends of the coil so as to modify the acoustic radiation diagram as a function of the frequency.
According to another embodiment, the motive device comprises at least one coil formed on a cylindrical wall portion of a spindle, which creates an anisotropic magnetic pressure exerted on at least a cylindrical portion of the tube.
The coil can comprise a core of high magnetic permeability and of low coercive force.
The electrical generator can be a shock generator suited to provide current pulses or a generator suited to provide current pulse trains so as to generate vibrations in the medium. This pulse train generator can for example be controlled by a control element suited to generate a variable-frequency control signal.
The tube is for example a well casing tube mechanically coupled with the formations surrounding the well and the motive device can comprise a sonde connected to an electric excitation generator, the sonde moving in the well up to the triggering point.
According to an embodiment, the electrical generator comprises for example a battery comprising capacitors arranged in an enclosure in the vicinity of the sonde, supplied by an electrical source remote from the enclosure.
According to an embodiment, the tube is the lateral wall of a sealed enclosure, the motive device comprising a coil placed in the enclosure, the electrical generator being at least partly outside the enclosure.
The device can be used for example within the scope of operations of onshore seismic exploration or monitoring of an underground reservoir. Waves are emitted in the formations surrounding the well by triggering an electrical generator. Waves reflected by the underground zone discontinuities are received and recorded. The records are processed so as to form seismograms of the zone.
The device can also be used for example within the scope of operations of marine seismic exploration or monitoring of an underground zone below a water body, with immersion of an enclosure from a vehicle or a stationary installation. Waves in the water body are emitted by triggering electrical generator. Waves reflected by the underground zone discontinuities are received and recorded. The records are processed to form seismograms of the zone.